Pickling is a process in stainless steel production that refers to treating the steel with a mixture of acids to remove metal oxides and chrome-depleted layers from the steel. For instance, stainless steel is usually pickled with a mixture of nitric/hydrochloric/hydrofluoric acid or a combination of sulfuric acid/hydrofluoric acid. This results in an acidic solution containing various metals (such as nickel, chromium and iron) and when the amounts of dissolved metals are too high, the pickling acids loses their effectiveness.
The primary acid used for pickling is hydrochloric acid, but a combination of various acids such as hydrofluoric, nitric, hydrochloric and sulfuric acid has also been a common choice. In automatic steel mills, hydrochloric acid is used as it pickles much faster while minimizing metal loss. Carbon steels with less than 6% alloy content are generally pickled in hydrochloric or sulfuric acids, while higher alloy content steels require a two-step pickling process using acids such as nitric, phosphoric or hydrofluoric acid. For rust and acid resistant steels, a pickling bath of nitric and hydrochloric acid is used.
Several different technologies have been developed to recycle the pickling acids and to recover the dissolved metals for re-use. Patent GB2036573 discloses the process of distillation of volatile acids in the presence of sulfuric acid and the recovery of the volatile acids by means of condensation. In a similar way, publication WO95/04844 discloses a process where volatile acids containing metal salt impurities, such as metal pickling solutions, are regenerated by a process in which the acid is subjected to sulfuric acid distillation. The resulting volatile acid vapor is condensed and recycled to the pickle tank, while the residual acid mixture is treated in an acid absorption unit, preferably of the acid retardation type. Acid absorbed in the acid absorption unit is periodically eluted with water and recycled, while metal impurities, in the form of sulfates are rejected, typically neutralized and discarded.
Publication WO2010/051992 discloses a process for complete recovery of metals and acids from solutions and sludge coming from stainless steel works. Volatile acids are recovered though distillation in the presence of sulfuric acid, while the metals are separately recovered in the form of hydroxides or salts. The metals are recovered by neutralization and precipitation with sodium hydroxide.
Current recovery efforts include ionic exchange methods, where metal ions are absorbed by an anionic bed, thus allowing the acids to be recycled back into the process. The metals can then be released with sedimentation and microfiltration of the bed, then regenerating the bed with water. Using this method, the need for new acids is reduced by 30 to 50% depending on the acid.
At Outokumpu steel mill in Tornio, Finland, a method for recycling of nitric and hydrofluoric acid uses evaporation, where sulfuric acid is first mixed into the pickling liquor, which in turn is then evaporated under vacuum at 80° C. The nitric and hydrofluoric acids evaporate and can be recycled, and the remaining material is a cake of metal sulfates and free sulfuric acid.
A common problem with metal recovery from pickling acid residues is filtration, as hydroxides tend to bind water and thus create a gel, making filtration difficult or impossible. Ferrous hydroxides are generally the most abundant hydroxides in pickling acid residues, and in European Patent Application Publication No. EP0161050, a method using solvent extraction to selectively remove Fe3+ is described. This method, while effective in removing iron from the pickling waste, still leaves other metal ions in the solution, which is now very difficult to treat due to residual liquid from the solvent extraction.
Typically, the solution is treated with lime to neutralize the acids and the material is then landfilled. Alkali neutralization is commonly done with limestone, either as calcium carbonate, calcium oxide or calcium hydroxide. Alternatively, sodium hydroxide and potassium hydroxide can be used and are faster acting, requiring only about 5-10 minutes to fully neutralize any pickling waste in contrast to about 30 minutes when neutralized with limestone. While NaOH and KOH are more expensive chemicals, less is required, so both time and material is saved. However, limestone neutralization gives better sedimentation and dewatering properties of the formed hydroxides, reducing the chance of metal seepage into the environment. The divalent charge of calcium is the primary reason for this, as the ionic strength of the bond becomes more powerful.
To recover both the acids and the metals would be an important step towards a more economical and environmentally friendly process. The residue contains valuable metals in consistent concentrations, and could potentially be processed into metal products or used as raw material for other processes. Potentially the residue could be processed into raw material for the steel industry according to US patent publication 2011/0308352A1, where a mix of iron, chromium and nickel is made into a ferrochrome nickel product that can be used for stainless steel production.